In sewing, sewing conditions vary depending on how tension of a lower thread is adjusted. Particularly, if tension of an upper thread is too great, the lower thread would be pulled out over a fabric, while, if the tension of the upper thread is too small, thread tightness becomes insufficient, which would result in a bad-looking stitch. Therefore, it has heretofore been conventional to perform sewing operation while appropriate adjusting the tension of the upper thread. Patent Literature 1 identified below, for example, discloses a technique which detects tension of the upper thread by means of an upper thread tension sensor and adjusts the tension of the upper thread on the basis of the thus-detected tension value so as to control the upper thread tension and thereby achieve a desired sewing finish. Non-patent Literature 1 identified below, on the other hand, discloses analyzing a rate of stitch tightness by skeleton-modeling a stitch structure and then deriving relationship between the rate of stitch tightness and the upper thread tension. Further, Patent Literature 2 and Patent Literature 3 identified below disclose a technique which achieves a desired sewing finish by calculating a consumed quantity of the upper thread (upper thread consumption quantity) per stitch on the basis of a stitch length corresponding to a desired embroidery pattern, fabric thickness and target stitch tightening allowance and then performing compulsory upper thread pay-out control using the calculated upper thread consumption quantity as a target value. In other words, the inventions disclosed in Patent Literature 2 and Patent Literature 3 are each arranged to, on the basis of the principles disclosed in Non-patent Literature 1, pre-calculate an ideal pay-out quantity per stitch of the upper thread and perform the compulsory upper thread pay-out control corresponding to the pre-calculated ideal pay-out quantity.
However, according to the disclosure of Patent Literatures 1 to 3 etc., no evaluation is made of degree of stitch tightness in an actually sewn product (or actual finished sewn product). Further, the technique disclosed in Non-patent Literature 1 too merely analyzes the relationship between the rate of stitch tightness and the upper thread tension and does not determine or evaluate acceptability/non-acceptability of the degree of stitch tightness in the actually sewn product. Particularly, when the upper thread has failed to be captured by a hook, there would occur stitch skipping and hence a defective stitch or stitches, or when a breakage has occurred in the upper and/or lower thread, a detective product would result if such a breakage is overlooked. Thus, the conventionally-known techniques cannot inspect such a defective stitch and defective product.